This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Objective: In recent years there has been increased interest to perform cardiac interventions such as EP ablation under MR-guidance. Directly monitoring the temperature rise during these procedures could potentially be helpful to verify successful ablation and predict treatment outcome. Here, we investigate the feasibility of monitoring temperature changes in the left ventricular myocardium in real-time. Temperature images based on the proton resonance frequency (PRF) shift are reconstructed using a hybrid method that combines multi-baseline subtraction and referenceless thermometry. Materials and Methods: Short-axis free-breathing cardiac images were acquired in three volunteers (no heat applied) in real-time using spiral gradient echo acquisitions with 4-5 interleaves on a 3T scanner using echo times of 3 ms, 5 ms, and 7 ms. Hybrid temperature image reconstruction was performed off-line in Matlab. The hybrid imaging model assumes that three sources contribute to image phase during thermal treatment: Background anatomical phase, spatially smooth phase deviations, and focal, heat-induced phase shifts. For the referenceless portion of the processing, sixth-order background polynomials were used and the multi-baseline libraries were comprised of 150 images (sliding window reconstruction) acquired during free breathing, representing approximately three cardiac cycles. Temperature reconstruction was performed over circular regions of interest containing the entire left ventricle. Temperature uncertainty was measured in the septum and the free ventricular wall in images during systole and diastole. To read about other projects ongoing at the Lucas Center, please visit http://rsl.stanford.edu/ (Lucas Annual Report and ISMRM 2011 Abstracts)